1. Field of the Invention
The invention relates to microwave drying. Specifically, the invention relates to drying ink with microwave energy during ink jet printing.
2. Description of the Related Art
In color ink jet printing, a relatively large quantity of ink is deposited onto the print media in a relatively short period of time. Often, there is a significant time period between the completion of a portion of an image and ink drying in that portion. In some cases, a printed image may be ruined by being rolled onto a take up reel on the printer after the image is printed but before the ink is dry. This is an especially apparent problem in humid environments, where ink drying times are considerably extended.
Furthermore, in multi-pass ink jet printing, the print head is passed over the same part of the media several times, with a portion of the required droplets deposited with each pass. In these types of print operations, quality is improved if the ink deposited in the previous pass is sufficiently dry before the print head is passed over the same part of the media a subsequent time.
To help alleviate problems associated with slow ink drying rates, various methods of drying the ink during or after printing have been developed. Some of these methods involve heating various printer components with infrared radiation or by directing heated air onto the media. These methods are inefficient at coupling heat to the printed media. In addition, water based ink can be heated by microwaves and microwave drying systems to heat and dry the deposited ink have been designed. These systems operate at about 2.45 GHz, an allowed industrial band. One such system is described in U.S. Pat. No, 5,220,346 to Carriera et al. In this system, the media is fed through a stationary microwave dryer after the ink is deposited. The dryer essentially comprises a waveguide with a magnetron and tuner coupled to one end. At least some of the microwaves in the waveguide are absorbed by the ink as the media passes through, thereby heating and drying the ink.
This type of system suffers from various difficulties. The first is that with 600 watts applied, the resultant electric fields are only about 3xc3x97104 volts/meter. A second is the fact that different portions of the cavity have different average electric field intensities, and so the drying is uneven across the image. Furthermore, even if a constant field intensity across the image were to be produced, different ink densities on different image portions will also cause uneven drying.
Image quality defects are also associated with the relatively large amount of liquid deposited on the media. For example, heavy liquid deposition can cause image defects such as color bleed, coalescence and paper deformation known as cockle. It is impossible to control coalescence with U.S. Pat. No. 5,631,685 because the print media is not dried until after the print media leaves the printer.
Additional examples of microwave drying apparatus include U.S. Pat. No. 5,631,685 awarded to Arthur Gooray. The printer described in this patent passes ink jet printed sheets through multiple applicator sections to dry the ink with a dryer similar to the low electric field apparatus described in U.S. Pat. No. 5,220,346 assigned to Carriera et al. This stationary microwave drier is bulky and still requires the sheet to leave the printer for drying. Thus, while a goal is to control cockle, the delay between printing and drying in the stationary microwave applicator makes it impossible to completely control cockle.
As another example, U.S. Pat. No. 4,234,775 awarded to Wolfberg and Harper describes a system wherein the electric field strength for web or sheet drying is enhanced by creating resonant zones of standing waves in a waveguide, then using multiple waveguides with xc2xc xcex offsets to achieve uniformity of drying. However, unevenness in drying still results and the device is large and bulky.
Thus, the state of the art of microwave drying for ink jet printers and for web, sheet or film drying in general is to utilize low electric field applicators that are bulky or to utilize higher electric field, resonant devices that use a phase shifting or offset geometry in an attempt to achieve an average uniformity.
In one embodiment of the invention, a microwave energy applicator comprises a first cavity having a first opening therein, a second cavity having a second opening therein, and a substantially conductive barrier defining a boundary between the first and second cavities and the first and second openings.